The present invention relates generally to a reduction projection lens system for use on equipment for the fabrication of integrated circuits such as ICs and LSIs, and more particularly to a reduction projection lens system for transferring a circuit pattern drawn on a mask onto a silicon wafer by the reduction projection aligning process.
Generally, the resolution of an image projected with a projection lens system is proportional to the numerical aperture and inversely proportional to the wavelength used. With recent remarkable progress in the degree of integration of integrated circuit, there is an increasing need for projection lens systems having some increased resolving power. Resolving power becomes better with an increase in numerical aperture. However, this makes the depth of focus so small that it is required to place focusing under very precise control. There is another problem that a silicon wafer onto which a circuit pattern is to be transferred is required to have very high-flatness, and which becomes unpractical. In recent years, it has thus been attempted to increase resolving power while the depth of focus is maintained by making the wavelength used short rather than by making the numerical aperture large. At present, light of 436 nm or 365 nm from mercury lamps has been used. Light sources of 300 nm or shorter wavelengths are also used. For instance, JP-A 60-140310 teaches the use of a KrF excimer laser that gives out an emission spectrum of 248 nm, and JP-A 1-315709 and 5-34593 refer to the use of ArF excimer lasers that give out an emission spectrum of 193 nm.
To correct for chromatic aberration of lens systems, the use of diffractive optical elements characterized by the inverse dispersion has been put forward, as set forth in JP-A 4 214516.
Essentially inevitable for a reduction projection lens system is to make substantially complete correction for both axial and off-axis aberrations, thereby assuring high resolution and a wide exposure region. However, there is an unavoidable increase in the number of lenses forming a projection lens system.
For instance, to correct for spherical aberration of a lens system consisting only of spherical lenses made of a single vitreous material, it is necessary to divide one lens into a plurality of lens elements which have the same power in total. To correct for field curvature, a number of lenses having positive and negative refracting powers must be arranged at proper locations, resulting in an extreme increase in the number of lenses forming the lens system.
The vitreous material usable at wavelengths of and shorter than 300 nm is practically limited to SiO.sub.2 (quartz) or CaF.sub.2 (fluorite) in view of transmittance. If other practical factors such as processability are taken into consideration, however, the usable vitreous material is restricted to SiO.sub.2 (quartz) alone. For instance, consider a reduction projection lens system using a KrF excimer laser of 246 nm wavelength as a light source and only SiO.sub.2 (quartz) as the vitreous material. As can be seen from JP-A 60-140310, there are increases in both the number of lenses and the total thickness of the vitreous material. Therefore, the use of a light source of shorter wavelengths, e.g., an ArF excimer laser of 193. 5 nm gives rise to a transmittance decrease even with the use of SiO.sub.2 (quartz). Thus, there have been some problems such as magnification and best focus variations of lenses due to exposure light and heat absorption, and low throughout due to underexposure.
To avoid this transmittance drop, it has been attempted to use aspheric surfaces for a projection lens system to reduce the total lens thickness, as set forth in JP-A 5-34593. In this case, however, it is impossible to correct for chromatic aberration of the lens system, because the lens system is built up of a single vitreous material. This has led to another problem that unless the bandwidth of the wavelength spectrum of the light source is extremely reduced, there is then a drop of the resolution of the projection lens system. For instance, the lens system disclosed in JP-A 5-34593 would malfunction and become practically useless, unless the bandwidth is reduced to .+-.0.5 pm (picometer) (FWHM) or lower. For KrF or ArF excimer lasers, however, it is generally recommendable to insert in the resonators a wavelength dispersion element such as an etalons or a grating, thereby reducing the spectral bandwidth. Indeed, it is still very difficult to achieve stable laser oscillation after the wavelength spectrum has been reduced to .+-.0.5 pm (FWHM).
Such a load on the light source may possibly be reduced by incorporating in the projection lens system several CaF.sub.2 (fluorite) pieces as chromatic aberration correctors. However, CaF.sub.2 (fluorite) is a material so soft that it is difficult to process, that is, it cannot be used as a practical lens material. To correct for chromatic aberration, JP-A 4-214516 discloses the use of a diffractive optical element characterized by the inverse dispersion, but it fails to provide a solution to the problems mentioned above due to an increase in the total lens thickness.